While transits of the 18th and 19th centuries gave astronomers a way to answer one of the biggest astronomical questions of the time – just how big is the Solar System – modern day astronomers use transits to search for planets outside of our Solar System.

As a planet passes in front of a star it temporarily blocks out a tiny portion of the star’s light, giving away its presence. The dip in starlight can be detected by sensitive telescopes such as Europe’s CoRoT and NASA’s Kepler space telescopes, which plot the reduction in light from the star in the form of a ‘light curve’.

Example of a light curve resulting from a planet transiting across the face of its star. Credit: CNES

There are some caveats, of course – the telescopes have to be looking in the line of sight of the planet as it transits across the face of its star, and to confirm the observation astronomers must wait for the planet to transit in front of the star several times. For example, if someone were watching the Earth transit across the face of the Sun, they would have to wait a year for each transit to occur. That’s why it can sometimes take several years for exoplanet ‘candidates’ to be confirmed.

Sometimes the light curves reveal that more than one planet is present around the star – of the 770+ planets known to date, over one hundred multiple planet systems are known!

As well as simply telling astronomers that a planet is present, the amount by which the brightness of the host star is reduced provides details of the planet’s size.

The transit method also makes it possible to study the atmosphere of the planet, and this is one of the techniques that astronomers will be testing during next week’s transit of Venus. As the planet passes in front of its host star, light from the star filters through the upper atmosphere of the planet. By studying the ‘fingerprints’ that the atmosphere leaves on the star’s light, astronomers can determine the concentration of different molecules in the planet’s atmosphere. Such a technique may enable astronomers to detect signs of life on potentially habitable Earth-sized worlds elsewhere in the Galaxy.